[0001] This invention relates to a selective silicidation process in the formation of non-volatile
electronic memory devices integrated on a semiconductor substrate, said non-volatile
memory devices comprising a plurality of active elements formed with gate and drive
regions comprising at least one polysilicon layer.
[0002] The invention relates in particular, but not exclusively, to a selective silicidation
process for the active elements of an EPROM or Flash EPROM cell, and the description
to follow will consider this field of application for convenience of explanation.
Prior Art
[0003] As is well known, non-volatile memory devices integrated on a semiconductor substrate
comprise:
- matrices of memory cells, wherein each cell comprises a floating gate MOS transistor;
and
- control circuitry comprising fast-logic MOS transistors.
[0004] Each floating gate MOS transistor conventionally comprises a drain region and a source
region which are formed in the semiconductor substrate and separated by a channel
region. A floating gate electrode is formed over the substrate and isolated therefrom
by a thin layer of gate oxide.
[0005] A control electrode is coupled capacitively to the floating gate electrode through
a dielectric layer.
[0006] Each matrix of memory cells is organized into rows, known as the word lines, and
columns, known as the bit lines. Cells belonging to the same word line have a common
supply line which drives their respective control electrodes, and cells belonging
to the same bit line have their drain terminals in common.
[0007] Where memory devices of very small size are to be formed, the interconnect lines
between the gate electrodes, which may be on the order of 0.25µm, for example, comprise
layers of a low resistivity material.
[0008] A first prior solution for providing such low resistivity material layers has been
the use of composite material layers, known as silicides, which comprise silicon and
a transition metal such as titanium, to cover those areas where resistivity is to
be lowered.
[0009] The formation of a silicide layer over the active areas of MOS transistors comprises
the following steps, following the formation of the transistor gate:
- implanting source and drain regions of the transistor;
- depositing a transition metal;
- subjecting the transition metal to a thermal process for selectively reacting it with
the substrate surface and producing the silicide layer.
[0010] While being in several ways advantageous, this first prior solution still has certain
drawbacks.
[0011] In fact, in applying the thermal process for reacting the transition metal layer
with the substrate surface, thereby to produce the silicidation of the implanted source
and drain regions, a surface layer of the substrate is consumed, and some of the dopant
in the substrate leaks into the silicide layer. Accordingly, the silicide layer may
become short-circuited to the substrate, thereby interfering with the normal operation
of the cell.
[0012] The underlying technical problem of this invention is to provide a silicidation process
for the gate electrodes of electronic devices, specifically memory devices, having
such features that the formation of a silicide layer over the implanted regions in
the semiconductor substrate can be prevented, thereby overcoming the drawbacks with
which prior art silicidation processes have been beset.
[0013] Silicidation processes are known from US-A-5 683 941, US-A-5 470 772 and EP-A-0 200
364.
Summary of the Invention
[0014] The solving idea behind this invention is that of providing an improved process flow
for silicidizing electronic devices which comprises, before the silicidation step,
a step of forming a dielectric layer to cover all the areas where the silicidation
process may be critical and only leave uncovered those polysilicon portions where
silicidation is instead desirable, without introducing any additional masking steps
in the process.
[0015] Based on this solving idea, the technical problem is solved by a silicidation process
as defined in Claim 1.
[0016] The features and advantages of a process according to the invention will be apparent
from the following description of an embodiment thereof, given by way of non-limitative
example with reference to the accompanying drawings.
Brief Description of the Drawings
[0017] In the drawings:
Figures 1 to 10 are respective enlarged vertical cross-section views of a portion
of a semiconductor substrate as a silicidation process according to the invention
progresses.
Detailed Description
[0018] Referring to the drawing views, an improved selective silicidation process for electronic
devices 1 integrated on a semiconductor substrate 2, specifically memory devices of
the EPROM and Flash EPROM types, will be described herein below.
[0019] Shown to an enlarged scale in the Figures are vertical cross-sections through portions,
not necessarily adjacent one another, of a semiconductor substrate wherein the memory
devices are formed in accordance with the invention.
[0020] Some process steps will not be discussed in detail in the description which follows,
not to burden it with information of minor weight already available to the skilled
persons in the art.
[0021] The first step in the process of fabricating EPROM or Flash EPROM memory devices
conventionally comprises forming active areas 8 of all the active elements 3, such
as floating gate MOS transistors 3b, of the matrix, and transistors 3a of the circuitry.
[0022] The definition of the active areas 8, isolated from one another by insulating regions
consisting of a field oxide 9, is carried out conventionally.
[0023] Subsequently to defining the active areas 8, a gate oxide layer 10 may be grown for
forming the transistors 3a, 3b.
[0024] A first conducting layer 11, e.g. of polysilicon, is then deposited.
[0025] A first resist mask for patterning the first polysilicon layer 11, referred to in
the art as the POLY1 mask, is used to define the floating gate electrodes 4a of the
memory cells.
[0026] By way of a conventional photolithographic step, the unprotected polysilicon layer
by the POLY1 mask is etched away.
[0027] Upon removal of the POLY1 mask, an intermediate dielectric layer 12, e.g. of ONO
(Oxide-Nitride-Oxide), is deposited.
[0028] A second resist mask, referred to in the art as the MATRIX mask, is then used for
masking the portion of the semiconductor where the memory matrix is to be formed.
[0029] By a conventional photolithographic step, the first layer of polysilicon 11 and intermediate
dielectric 12, being unprotected by the MATRIX mask, are etched away to expose the
gate oxide layer 10 in the portion of the semiconductor where the circuitry is being
formed.
[0030] A second conductive layer, e.g. of polysilicon, is then deposited over the entire
substrate.
[0031] At this step of the fabrication process, the control electrodes 4b and word lines
WL of the cell matrix which will drive the control electrodes of the memory cells
in the same column are defined.
[0032] A third resist mask, referred to in the art as the self-aligned etch mask and used
for defining the second polysilicon layer 12, is then used for defining the word lines
WL of the cell matrix.
[0033] The second layer of polysilicon 5, unmasked by the self-aligned etch mask, is etched
away by a conventional photolithographic step.
[0034] So the floating gate electrodes 4 of the memory matrix cells have been formed.
[0035] Advantageously, before removing the self-aligned etch mask, a fourth mask, referred
to in the art as the SAS mask, is provided.
[0036] By a conventional photolithographic step, the residual field oxide layer between
the word lines WL is etched away as shown in Figure 7, this being followed by an ion
implantation step to form the source and drain regions 13.
[0037] The process is continued with the deposition of a first dielectric layer 6 over the
entire surface of the substrate. For example, this dielectric layer 6 could be of
the TEOS type.
[0038] The thickness of the dielectric layer is illustratively within the range of 500Å
to 3000Å, enough to isolate the transistor gate electrodes 4 and the word lines of
the matrix.
[0039] A second dielectric layer 6a is deposited next to fill the gaps between the word
lines of the matrix.
[0040] This second dielectric layer may be a doped insulating oxide film of the BPSG type.
[0041] Alternatively, this second dielectric layer may be deposited, by a HDPCVD (High Density
Plasma Chemical Vapor Deposition) technique, or a SACVD (Sub-Atmospheric Chemical
Vapor Deposition) technique.
[0042] Nothing prevents from using a single dielectric filler layer 6 to fill the gaps between
the word lines of the matrix.
[0043] At this point, the inventive process provides for a removal of the second dielectric
layer 6b by a CMP (Chemical Mechanical Polishing) treatment to produce mechanically
a planar surface.
[0044] This removal of second dielectric layer could also be achieved by a dry etch-back
step.
[0045] The etch-back is carried out to advantage using no additional masks, and is continued
until the surface of the second polysilicon layer 5 of the word lines and the surface
of the second polysilicon layer in the circuitry become exposed.
[0046] In this way, the second dielectric layer is confined to just the gap regions between
the word lines, and only the areas to be silicized are exposed.
[0047] At this point, a transition metal, such as titanium, is deposited and then processed
thermally to cause it to react selectively with the uncovered areas by the dielectric
layer and produce a silicide layer 7.
[0048] In summary, the process of this invention allows the resistance of the conductive
polysilicon layers to be reduced, and silicidation of the source and drain regions
of the memory matrix avoided, thereby preventing any dangerous bridging by the silicide
between the source/drain regions and the first polysilicon layer or the substrate
surface in the active areas, which would impair the proper operation of the matrix,
without introducing any additional masking steps in the process flow.
1. A selective silicidation process in the formation of non-volatile electronic memory
devices (1) integrated on a semiconductor substrate (2), said non-volatile memory
devices (1) comprising a memory matrix and a circuitry, the memory matrix comprising
a plurality of active elements (3) formed with gate (4) and drive (WL) regions, the
process comprising the following steps in the following order :
- forming active areas of said active elements (3),
- forming a gate oxide layer (10) on the semiconductor substrate (2),
- forming a first conducting layer (11) on said gate oxide layer (10),
- patterning the first conducting layer (11) with a first resist mask,
- etching the first conducting layer (11) that is unprotected by the first resist
mask to form floating gate electrodes (4a) of the memory matrix,
- forming an intermediate dielectric layer (12) on the entire surface of the semiconductor
substrate (2),
- masking the memory matrix with a second resist mask,
- etching the intermediate dielectric layer (12) and the first conducting layer (11)
that is unprotected by the second resist mask,
- forming a polysilicon layer (5) on the entire surface of the semiconductor substrate
(2),
- patterning the polysilicon layer (5) with a third resist mask,
- etching the polysilicon layer (5) that is unprotected by the third resist mask to
form control gate electrodes (4b) and word lines of the memory cells,
- forming source and drain regions (13) of the memory cells,
- depositing a dielectric layer (6) on the entire surface of the semiconductor substrate
(2);
- removing said dielectric layer (6) to expose the polysilicon layer (5);
- depositing a layer of a transition metal (7);
- subjecting the transition metal layer to a thermal treatment for selectively reacting
it with the polysilicon layers and producing a silicide layer (8) on said control
gate electrodes (4b), said drive regions and said second polysilicon layer (5) in
the circuitry.
2. A selective silicidation process according to Claim 1, characterized in that the step of depositing said dielectric layer (6) comprises a first step of depositing
a first dielectric layer of thickness enough to isolate the control gate electrodes
(4b) and a second step of depositing a second dielectric layer to fill the gaps between
the word lines of the memory matrix.
3. A selective silicidation process according to Claim 1, characterized in that before removing the third mask, a fourth mask is deposited.
4. A selective silicidation process according to Claim 1, characterized in that said removing step is a planarizing step.
5. A selective silicidation process according to Claim 4, characterized in that said planarizing step is carried out by CMP.
6. A selective silicidation process according to Claim 4, characterized in that said planarizing step is carried out as a dry etch-back step.
7. A selective silicidation process according to Claim 1, characterized in that the dielectric layer is TEOS.
8. A selective silicidation process according to Claim 1, characterized in that the metal layer is titanium.
9. A selective silicidation process according to Claim 2, characterized in that the first dielectric layer is TEOS and the second dielectric layer is BPSG.
1. Selektives Silizidierungsverfahren bei der Bildung von nicht flüchtigen, elektronischen
Speicherbauelementen (1), die auf einem Halbleitersubstrat (2) integriert sind, wobei
die nicht flüchtigen Speicherbauelemente (1) eine Speichermatrix und eine Schaltung
aufweisen, wobei die Speichermatrix eine Vielzahl von aktiven Elementen (3) umfasst,
die mit einem Gatebereich (4) und einem Treiberbereich (wl) ausgebildet sind, wobei
das Verfahren folgende Schritte in folgender Reihenfolge umfasst:
- Bilden von aktiven Zonen der aktiven Elemente (3),
- Bilden einer Gate-Oxidschicht (10) auf dem Halbleitersubstrat (2),
- Bilden einer ersten leitenden Schicht (11) auf der Gate-Oxidschicht (10),
- Strukturieren der ersten leitenden Schicht (11) mit einer ersten Ätzschutzmaske,
- Ätzen der ersten leitenden Schicht (11), die von der ersten Ätzschutzmaske nicht
geschützt ist, um Elektroden (4a), mit schwebendem Gate, der Speichermatrix zu bilden,
- Bilden einer dazwischen liegenden dielektrischen Schicht (12) auf der gesamten Oberfläche
des Halbleitersubstrats (2),
- Maskieren der Speichermatrix mit einer zweiten Ätzschutzmaske,
- Ätzen der dazwischen liegenden dielektrischen Schicht (12) und der ersten leitenden
Schicht (11), die von der zweiten Ätzschutzmaske nicht geschützt ist,
- Bilden einer Polysiliziumschicht (5) auf der gesamten Oberfläche des Halbleitersubstrats
(2),
- Strukturieren der Polysiliziumschicht (5) mit einer dritten Ätzschutzmaske,
- Ätzen der Polysiliziumschicht (5), die von der dritten Ätzschutzmaske nicht geschützt
ist, um Steuergate-Elektroden (4b) und Wortleitungen der Speicherzellen zu bilden,
- Bilden von Source- und Drainbereichen (13) der Speicherzellen,
- Abscheiden einer dielektrischen Schicht (6) auf der gesamten Oberfläche des Halbleitersubstrats
(2);
- Entfernen der dielektrischen Schicht (6), um die Polysiliziumschicht (5) freizulegen;
- Abscheiden einer Schicht aus einem Übergangsmetall (7);
- Unterwerfen der Schicht aus Übergangsmetall einer Wärmebehandlung, um sie selektiv
mit den Polysiliziumschichten zur Reaktion zu bringen, und Herstellen einer Silizidschicht
(8) auf den Steuergate-Elektroden (4b), den Treiberbereichen und der zweiten Polysiliziumschicht
(5) in der Schaltung.
2. Selektives Silizidierungsverfahren nach Anspruch 1, dadurch gekennzeichnet, dass der Schritt des Abscheidens der dielektrischen Schicht (6) einen ersten Schritt des
Abscheidens einer ersten dielektrischen Schicht umfasst, deren Dicke groß genug ist,
um die Steuergate-Elektroden (4b) zu isolieren, und einen zweiten Schritt des Abscheidens
einer zweiten dielektrischen Schicht, um die Zwischenräume zwischen den Wortleitungen
der Speichermatrix aufzufüllen.
3. Selektives Silizidierungsverfahren nach Anspruch 1, dadurch gekennzeichnet, dass vor dem Entfernen der dritten Maske eine vierte Maske abgeschieden wird.
4. Selektives Silizidierungsverfahren nach Anspruch 1, dadurch gekennzeichnet, dass der Schritt des Entfernens ein Planarisierungsschritt ist.
5. Selektives Silizidierungsverfahren nach Anspruch 4, dadurch gekennzeichnet, dass der Planarisierungsschritt mittels CPM ausgeführt wird.
6. Selektives Silizidierungsverfahren nach Anspruch 4, dadurch gekennzeichnet, dass der Planarisierungsschritt als Trockenätzschritt ausgeführt wird.
7. Selektives Silizidierungsverfahren nach Anspruch 1, dadurch gekennzeichnet, dass die dielektrische Schicht aus TEOS besteht.
8. Selektives Silizidierungsverfahren nach Anspruch 1, dadurch gekennzeichnet, dass die Metallschicht aus Titan besteht.
9. Selektives Silizidierungsverfahren nach Anspruch 2, dadurch gekennzeichnet, dass die erste dielektrische Schicht aus TEOS besteht und die zweite dielektrische Schicht
aus BPSG.
1. Procédé sélectif de formation d'un siliciure lors de la formation de dispositifs de
mémoire électronique non volatile (1) intégrés sur un substrat semiconducteur (2),
lesdits dispositifs de mémoire non volatile (1) comprenant une matrice de mémoire
et un circuit, la matrice de mémoire comportant une pluralité d'éléments actifs (3)
formés avec la région de grille (4) et la région de commande (WL), le procédé comprenant,
dans l'ordre suivant, les étapes consistant à :
- former des zones actives desdits éléments actifs (3),
- former une couche d'oxyde de grille (10) sur le substrat semiconducteur (2),
- former une première couche conductrice (11) sur ladite couche d'oxyde de grille
(10),
- structurer la première couche conductrice (11) à l'aide d'un premier masque de resist,
- attaquer chimiquement de la première couche conductrice (11) qui n'est pas protégée
par le premier masque de resist pour former des électrodes de grille flottantes (4a)
de la matrice de mémoire,
- former une couche diélectrique intermédiaire (12) sur l'ensemble de la surface du
substrat semiconducteur (2),
- masquer la matrice de mémoire avec un second masque de resist,
- attaquer chimiquement la couche diélectrique intermédiaire (12) et la première couche
conductrice (11) qui n'est pas protégée par le second masque de resist,
- former une couche de polysilicium (5) sur l'ensemble de la surface du substrat semiconducteur
(2),
- structurer la seconde couche de polysilicium (5) avec un troisième masque de resist,
- attaquer chimiquement la seconde couche semiconductrice (5) qui n'est pas protégée
par le troisième masque de resist de manière à former des électrodes de grille de
commande (4b) et les lignes de mots des cellules de mémoire,
- former des régions de source et de drain (13) des cellules de mémoire,
- déposer une couche diélectrique (6) sur l'ensemble de la surface du substrat semiconducteur
(2),
- éliminer ladite couche diélectrique (6) pour exposer la couche de polysilicium (5);
- déposer une couche d'un métal de transition (7);
- soumettre la couche de métal de transition à un traitement thermique pour la faire
réagir sélectivement avec les couches de polysilicium et produire une couche de siliciure
(8) sur ladite électrode de grille de commande (4b), lesdites régions de commande
et ladite seconde couche de polysilicium (5) dans le circuit.
2. Procédé sélectif de formation d'un siliciure selon la revendication 1, caractérisé en ce que l'étape de dépôt de ladite couche diélectrique (6) comprend une première étape de
dépôt d'une première couche diélectrique ayant une épaisseur suffisante pour isoler
les électrodes de grille de commande (4b) et une seconde étape de dépôt d'une seconde
couche diélectrique pour remplir les interstices entre les lignes de mots de la matrice
de mémoire.
3. Procédé sélectif de formation d'un siliciure selon la revendication 1, caractérisé en ce qu'avant le retrait du troisième masque, on dépose un quatrième masque.
4. Procédé sélectif de formation d'un siliciure selon la revendication 1, caractérisé en ce que ladite étape de retrait est une étape de planarisation.
5. Procédé sélectif de formation d'un siliciure selon la revendication 4, caractérisé en ce que ladite étape de planarisation est exécutée au moyen du procédé CMP.
6. Procédé sélectif de formation d'un siliciure selon la revendication 4, caractérisé en ce que ladite étape de planarisation est exécutée sous la forme d'une étape de rétro-corrosion
à sec.
7. Procédé sélectif de formation d'un siliciure selon la revendication 1, caractérisé en ce que la couche diélectrique est formée de TEOS.
8. Procédé sélectif de formation d'un siliciure selon la revendication 1, caractérisé en ce que la couche métallique est formée de titane.
9. Procédé sélectif de formation d'un siliciure selon la revendication 2, caractérisé en ce que la première couche diélectrique est formée par du TEOS et la seconde couche diélectrique
est formée par du BPSG.